Electrical sensing of cardiac signals is widespread and common in the diagnosis and treatment of cardiac disease. Cardiac signals are sensed in the form of ElectroCardioGrams (ECG) for diagnostic and monitoring purposes by devices such as conventional ECG strip chart recorders, signal averaged ECG monitors, Holter monitors, vector cardiographs, and a variety of similar devices. Cardiac signals are also sensed by most cardiac stimulation devices that apply electrical energy to the heart for therapeutic purposes, such as defibrillators and pacemakers.
The need to protect the electrical-circuits that sense the cardiac signals from potentially damaging electrical stimulation has long been recognized. Most cardiac stimulation devices, and all implantable cardiac stimulation devices such as implantable cardioverter defibrillators (ICDs) and pacemakers include some form of protection circuitry that can block unintended electrical energy to prevent it from damaging the relatively delicate sensing circuitry.
In addition to the need for protection from the higher therapeutic energy levels applied to the heart, there is a competing need for high sensitivity when sensing cardiac signals in order to closely evaluate the subtle differences between electrical levels created by normal and abnormal heart activity.
Many cardiac devices apply therapeutic electrical energy to the heart and then act to sense the resulting cardiac electrical activity to determine the effect of the therapeutic application. These devices include those used for defibrillation or pacing for example. For these devices, it is not necessary to start sensing immediately after delivery of, electrical energy in order to determine whether the heart is responding appropriately to the delivered therapy. It has generally been considered acceptable, for therapeutic purposes, that the cardiac sensing circuitry only needs to be able to sense the next cardiac event that occurs from 100 to 1500 ms following energy delivery.
In order to best meet these competing needs in the context of cardiac stimulation devices, the dual concepts of shunting and blanking have been developed. Shunting involves inserting switches between the conductors in the leads to the sensing circuitry to shunt or short the two leads together to prevent damage to the sensing circuitry by preventing any electrical energy from getting to the sensing circuitry.
Blanking involves selectively ignoring whatever signals are presented to the cardiac sensing circuitry for a given period of time following delivery of electrical stimulation energy. During the blanking period the sensing circuitry is isolated from cardiac electrical activity and no sensing can take place. As a result, existing cardiac sensing circuitry generally is not designed to allow for effective sensing of cardiac signals during the brief period of less than about 100 milliseconds immediately following delivery of electrical stimulation energy.
Recently, a technique has been developed to improve the ability of physicians to identify patients that are at an abnormally high risk for life-threatening cardiac arrhythmias. This technique, as described in U.S. Pat. Nos. 5,951,484 and 6,129,678, involves analyzing cardiac signals immediately after the application of relatively small levels of electrical stimulation that is below the stimulation threshold of cardiac tissue. During the period of the first 100 ms immediately following delivery of a subthreshold electrical stimulus, it is possible to observe minute electrical phenomenon in the sensed cardiac electrical activity such as the Wedensky phenomena. Conventional cardiac sensing circuitry is not designed to sense signals in this time period. So, the techniques described in these patents incorporate special cardiac sensing circuitry that uses fast recovery amplifiers in order to sense cardiac signals at high levels of sensitivity. Instruments using this approach can discriminate between sensing artifacts and true deviations in cardiac signals even though the cardiac tissue still has a residual electrical charge associated with the delivery of a subthreshold electrical stimulation.
While the technique of analyzing cardiac signals in response to the delivery of subthreshold electrical stimulation has the potential for significant improvement in the ability to determine patients' susceptibility to cardiac arrhythmias, it would be advantageous to be able to make use of these techniques in combination with conventional cardiac sensing circuitry.